Zero-IF (intermediate frequency) radio frequency (RF) front-end architectures are attractive for cellular systems due to lower cost and Bill-of-Material (BOM). A direct-conversion receiver (DCR), also known as homodyne, synchrodyne, or zero-IF receiver, is a radio receiver design that demodulates incoming signals by mixing it with a local oscillator signal synchronized in frequency to the carrier of the wanted signal. The wanted demodulated signal is thus obtained immediately by low-pass filtering the mixer output, without requiring further detection. The receiver has the advantage of high selectivity, and is inherently a precision demodulator.
However, an external SAW (surface acoustic wave) filter after the Low Noise Amplifier (LNA) stage has been an essential component of cellular systems for several reasons. The two main reasons for using an external SAW filter in full duplex communication systems, like CDMA and WCDMA, are Triple Beat (TB) and IIP2 performance requirements at the transmit (Tx) offset. (IIP2 is the theoretical input level at which the second-order two-tone distortion products are equal in power to the desired signals.) The calculation is based on the following equation: Tx power can leak to the LNA despite the duplexer isolation between the receive (Rx) and transmit (Tx) band. The typical duplexer isolation is 55 dB in the CDMA Cellular band, and the maximum Tx power can be as high as +27 dBm, resulting in −28 dBm of Tx power at the Rx input port. This strong Tx power can cause well-known cross-modulation distortion (XMD).
This distortion problem is usually dominated by the LNA performance, because Tx rejection by an external SAW filter reduces the mixer TB requirement. More importantly, the reduced Tx power at the mixer input reduces the IIP2 performance concern at the Tx offset frequency, since the second-order distortion at the Tx frequency offset can increase the noise floor of the receiver in a Zero-IF system.
A SAW-less receiver system is desirable since it eliminates the SAW filter as well as the external matching components. There have been several efforts to implement a SAW-less CDMA receiver. A Tx canceller used an LMS (least mean squares) adaptive filter requiring up/down conversion mixers and a low-pass filter in the loop. This method suffers from several performance problems. First, the noise figure (NF) of the receiver is degraded due to the operation of the LMS loop. Second, the rejection varies depending on the group delay of the external matching network. Third, the TB performance of the overall system can be degraded due to the LMS loop.
An on-chip Tx reject band-pass filter using bond-wire inductors for a WCDMA system has been reported. This method has benefits of saving area compare to an on-chip inductor and increasing the selectivity of filter due to high-Q of bond-wire. But this method may have limited feasibility in real production due to bond wire variations.